The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations, and particularly to polycrystalline diamond compact (PDC) drag bits.
Rotary bits of the kind to which the present invention relates comprise a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of preform cutting elements each formed at least in part from polycrystalline diamond, the gauge of the bit including a plurality of circumferentially spaced gauge pads which, in use, engage the surrounding formation forming the walls of the bore hole being drilled.
One common form of cutting element comprises a tablet, usually circular or part-circular, made up of a table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate of less hard material, usually cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in furnace so as to form a hard matrix.
Normally the majority of the cutting elements are mounted on a downwardly-facing end face of the bit body. However, some cutting elements, known as gauge cutters, may be mounted on certain of the gauge pads, such gauge cutters then determining the diameter of the bore hole being drilled. Commonly, cutting elements are mounted on only some of the gauge pads, certain of the gauge pads being free of cutting elements.
The gauge pads are subject to abrasion as the bit rotates in the borehole, and it is therefore normally considered necessary to provide some form of abrasion-resistant means on the gauge pads, for example in the form of abrasion resistant inserts or an abrasion resistant surface. However, the necessity of increasing the abrasion resistance of the gauge pads increases the cost of the bit, not only due to the cost of the abrasion resistant materials, but also due to the cost of the manufacturing process of applying such materials on the gauge pads. Generally speaking, the cost of rendering the gauge pads more abrasion resistant increases with the degree of abrasion resistance required.
Hitherto, it has been considered necessary to provide all the gauge pads of the drill bit with the same abrasion resistant means. According to the present invention, however, different forms of abrasion resistance are applied to different gauge pads of the drill bit, in a manner to reduce the overall cost of manufacture of the drill bit.